1. Field of the Invention
This invention relates to an apparatus for detecting a motion vector of a moving picture.
2. Description of the Relation Art
Motion compensation Inter-frame prediction is employed very frequently for high efficiency coding of a moving picture. For example, also in the International Standardization Organization ISO-IEC JTC1/SC2/WG11 (hereinafter referred to as MPEG), motion compensation inter-frame prediction is employed as an international standardization system, and for motion compensation, pels (picture elements) are divided into a plurality of blocks and motion compensation processing is performed in units of a block. The motion compensation processing is employed also in the international standardization system H.261 for moving picture coding for visual telephones. Further, motion compensation in units of a block is generally based on detection of a motion vector by block matching.
In the international standardization system H.261, a motion vector is detected by an accuracy of one pel (picture element), and one of the processors which can be utilized for such motion vector detection is, for example, STI3220 by SGS-THOMSON. Operation of the processor for such motion vector detection with block matching is described in Video Information Magazine, June 1991, pp. 83-89 or in "IMAGE PROCESSING DATA BOOK 2nd EDITION", SGS-THOMSON Microelectronics.
In the MPEG, improvement in coding efficiency is achieved particularly by raising the accuracy of a motion vector to be used for motion compensation to an accuracy of a half pel.
When it is tried to construct a motion vector detection apparatus conforming to the MPEG system, since the calculation processing amount and the memory capacity required for motion vector detection processing have exhibited a remarkable increase compared with those of the international standardization system H.261 in terms of the detection range and the detection accuracy, improvement for remarkable reduction of the calculation amount is required. One of the methods for simplification of motion vector detection is the reduction of candidate vectors by a multi-stage searching method. Such method is described in Japanese Patent Laid-Open Application No. Helsei 5-49023 (Japanese Patent Application No. Helsei 3-199614) which is a prior application of the assignee of the present invention in Japan. In the document, motion vector searching is performed such that motion vector searching in an accuracy of up to two pels is performed from the first stage to the K-1th stage and motion vector searching of another accuracy of one pel or less is performed at the Kth stage thereby to reduce the number of candidate vectors and the number of intra-block pels to be matched upon block matching up to the Kth stage to reduce the total amount of calculation processing.
Conventional motion vector detection apparatus require that the accuracies of pels of an object block for motion vector detection and a search area of a reference frame be set equal to each other in advance. Also in the processor STI3220 mentioned hereinabove, when it is tried to detect a motion vector in an accuracy of a half pel, since the accuracies of pels between blocks to be matched are different, pipeline processing between a vector less than the decimal point of a search area after interpolation of half pels and another vector of an integer is difficult.
In order to effect motion vector detection in an accuracy of a half pel which is required newly by the MPEG system, pels in a search area of a reference frame are interpolated by half pel interpolation in advance, the pels are divided into a group of original pels before interpolation, another group of pels is obtained by half pel interpolation in a horizontal direction, a further group of pels is obtained by half pel interpolation in a vertical direction, and a still further group of pels obtained by half pel interpolation in both 1 horizontal and vertical directions, and then, either parallel operations are performed by multiple chips, or 2 processing is performed for each of the pel groups, which requires that the processing be executed four times.